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356                           METALLURGY OF IRQ 1ST AND STEEL.
structural material the effect of this element is of little importance.
SEC. XYIIe.—Influence of phosphorus.—Of all the elements that are commonly found in steel, phosphorus is the most undesirable. In ordinary proportions its influence is not felt in a marked degree in the rolling mill, for it has no disastrous effect upon the toughness of red-hot metal when the content does not exceed .15 per cent. Its action upon finished material may not be dismissed in so few words. Prof. Howe* has gathered together the observations of different investigators, and the evidence seems to prove that the tensile strength is increased by each increment of phosphorus up to a content of .12 per cent., but that beyond this point the metal is weakened. Below this point it is certain that phosphorus strengthens lows steels, both acid and basic. The same certainty does not pertain to any other effect of this metalloid. Prof. Howef has discussed the whole matter, and I make quotations from The Metal--lurgy of Steel, in the form of a summary.
. (1)  The effect of phosphorus on the elastic ratio, as on elonga-tion and contraction, is very capricious.
(2) Phosphoric steels are liable to break under very slight tensile stress if suddenly or vibratorily applied.
(3)   Phosphorus diminishes the ductility of steel under a gradually applied load as measured hy its elongation, contraction and elastic ratio when ruptured in an ordinary testing machine, but it diminishes its toughness under shock to a still greater degree, and this it is that unfits phosphoric steels for most purposes.
(4)   The effect of phosphorus on static ductility appears to be very capricious, for we find many cases of highly phosphoric steel which show excellent elongation, contraction and even fair elastic ratio, while side by side with them are others produced under apparently identical conditions but statically brittle.
(5)   If any relation between composition and physical properties is established by experience, it is that of phosphorus in making steel brittle under shock; and it appears reasonably certain, though exact data sufficing to demonstrate it are not at hand, that phosphoric steels are liable to be very brittle under shock, even though they may be tolerably ductile statically.   The effects of phosphorus
* 'The Metallurgy of Steel, p. 67, et seq.      . t Loc. cit.